Changes

This is the same type of addon that Firefox currently uses. For a general introduction, see [https://developer.mozilla.org/docs/Mozilla/Add-ons/WebExtensions WebExtensions] and [https://developer.mozilla.org/docs/Mozilla/Add-ons/WebExtensions/What_are_WebExtensions What are WebExtensions].

However, the APIs are necessarily different between Thunderbird and Firefox. Some applicable Firefox APIs work in Thunderbird (just try it), other Firefox APIs do not work, and Thunderbird has a few Thunderbird-specific APIs added. There is some documentation for the [https://thunderbird-webextensions.readthedocs.io/en/latest/ Thunderbird WebExtension APIs].